700gish 1 season sythetic bags can be had for around £30, in fact my synthetic Karrimor was reduced to £13 in Blacks end of season sale. Tesco and Decathalon would be a good place to start for real budget bags. Tesco also do a summer down/feather/sythetic mix for about £40 iirc, but they seem to sell out fast.
One of the main issues with cheap sythetics is their pack size compared with down. For example my down summer bag is about 1/4 the packed volume of the Karrimor I mentioned above and has a slightly warmer rating, but then it cost 15 times as much.
Someone will be along soon to give you example of a more mid-range bag, but I tend to either buy cheap and cheerful or save up for very expensive.